1. Field of the Invention
This invention relates to a method and apparatus for decreasing nitrogen oxides discharged from a continuous combustion device of an external combustion engine, a boiler, etc. More particularly, the invention relates to a purification system for effectively purging nitrogen oxides by reduction without detracting from the excellent fuel-consumption characteristic of an engine such as a Stirling engine and without being influenced by the concentration of oxygen gas which coexists in the exhaust gas.
2. Description Of the Prior Art
In the operation of a Stirling engine, which is one type of external combustion engine, there is a process in which a working gas is heated and caused to expand by burning a fuel using a burner disposed in a combustion chamber. The heating of the working gas by combustion using a burner ordinarily utilizes the heat of combustion produced by supplying the interior of a combustion chamber with fuel and air for combustion and burning the fuel at high temperature. The exhaust gas resulting from combustion is discharged into the atmosphere through an exhaust passageway. In this case the fuel is completely combusted along with the combustion air at high temperature so that the exhaust gas will not contain any toxic components. However, even if combustion is performed under conditions in which the concentration of air is low (i.e., in which there is a low excess coefficient), the fact that exhaust gas resulting from continuous combustion will contain nitrogen oxides (NOx) cannot be avoided, even though the amount of uncombusted components HC and CO in the exhaust gas is low.
Methods of decreasing nitrogen oxides in exhaust gas produced by a continuous combustion device include a rarefied combustion method, a method which uses a ternary catalytic converter, etc. However, when a continuous combustion device of a Stirling engine or the like is considered, the air supplied in the rarefied combustion method is large in quantity and, hence, there is a decline in the combustion flame temperature. In addition, in order to increase the quantity of air supplied, there is an increase in the power consumed by an air-supply blower, thereby inviting a decline in engine efficiency. Accordingly, combustion usually is carried out at a low excess coefficient, namely an excess coefficient ranging from 1.1 to 1.3. As a consequence, this combustion method does not result in rarefied combustion and does not meet the conditions for decreasing nitrogen oxides.
For these reasons use of a ternary catalytic converter has been considered to be effective. In accordance with a method using a ternary catalytic converter, NOx, CO and HC present in exhaust gas are chemically converted into non-toxic components such as N.sub.2, CO.sub.2 and H.sub.2 O by a purifying catalyst. Since the chemical conversion proceeds at the same time as oxidation and reduction, it is required that the exhaust gas be maintained at a low oxygen concentration in order to raise the efficiency of reduction. In addition, the amount of change allowed in the oxygen concentration is very small. In order to employ this method, therefore, it is required that the amount of fuel and combustion air supplied be controlled with great accuracy. Properly speaking, the HC, and CO components in continuous combustion are low, even in case of a low excess coefficient, and therefore use of a ternary catalytic converter is not especially required. If a ternary catalytic converter is employed, it is necessary that the temperature of the exhaust gas be raised to 400.about.450.degree. C. or higher. When exhaust gas is elevated to a high temperature, preheating of the air by the exhaust gas cannot be carried out sufficiently and the temperature of the air supplied to the combustion section falls. This causes a decline in engine efficiency and is undesirable. Furthermore, since the exhaust gas exhibits a high temperature, there is a decline in the durability of the catalyst and an increase in cost since expensive precious metals must be used for the catalyst.